Use of bioelectrical impedance analysis and anthropometry to measure fat-free mass in children and adolescents with Crohn disease.

OBJECTIVES

To investigate the precision of published prediction equations for fat-free mass (FFM) from bioimpedance measurements in children with Crohn disease using dual-energy X-ray absorptiometry (DXA) as an in vivo gold standard.

METHODS

Fat-free mass of 49 white boys and girls ages 7.3 to 16.9 y suffering from Crohn disease was measured by DXA. Body weight, height and bioimpedance measurements were also collected. FFM measured by DXA (FFM(DXA)) was compared with FFM predicted by the only 5 published prediction equations available for children and adolescents. An equation was developed for predicting FFM and was validated using a bootstrap method.

RESULTS

When correlating predicted FFM with FFM(DXA), Schaefer's equation showed the highest R2 (0.950), the smallest standard error of estimate (SEE) (2.05 kg) and the smallest percentage error (0.28%). Our prediction equation for estimating FFM was FFM = 0.652 Ht2/Z + 0.0385 Wt + 0.586 Age - 0.327, R2 = 0.951, SEE = 2.08, P < 0.0005, where Ht2/Z is the impedance index in cm2/ohm, Wt is body weight in kilograms, age is in years. R2 value from bootstrap method was 0.950 +/- 0.01 (95% confidence interval 0.927-0.968), indicating an acceptable validation of the derived formula.

CONCLUSIONS

The formula of Schaefer is the best for predicting FFM. The present study provides a new prediction equation for estimating FFM in children with Crohn disease that may be used in clinical settings in which more sophisticated body composition measuring equipments are not available.